Process for preparing biodegradable water-resistant film and method for rendering biodegradable article water-resistant

ABSTRACT

A process for preparing a biodegradable water-resistant film. The process includes the steps of dispersing zein in an aqueous solution comprising acetone in an amount of from 65 to 75% by volume to form a zein dispersion, forming the resulting zein dispersion into a film, and drying the film in an atmosphere having a relative humidity of 85% or less at an initial temperature of 55 DEG  C. or less. The present invention also relates to a method for imparting water resistance to an article with poor water resistance by coating the article with the zein dispersion prepared as above, and drying the coated zein dispersion under the above conditions. The film or coating film according of the present invention neither dissolves upon contact with water, nor becomes weak due to water absorption or swelling on contact with water.

FIELD OF THE INVENTION

This invention relates to a process for preparing a water-resistantbiodegradable film and to application of the biodegradable film to abiodegradable but non-water-resistant article to render itwater-resistant.

BACKGROUND OF THE INVENTION

With the recent increase of consciousness of the importance ofenvironmental conservation and environmental purification, studies forreplacing conventional every-day materials with those made ofbiodegradable materials have been conducted on a world-wide scale.

Among biodegradable materials which have been proposed to date,starch-type naturally-occurring substances have poor water resistance;thus, their use is somewhat confined from a practical perspective.Imparting water resistance to such biodegradable materials would broadentheir application and furnish solutions to various problems confrontingthe environment, particularly in terms of waste disposal.

Articles with poor water resistance may be coated to waterproof them. Intaking this approach, it is required to establish a technique forproducing with ease a biodegradable thin film which exhibits sufficientwater resistance so as not to absorb water, not to become swollen withwater and not to be weakened even when contacted with water for a longtime. In particular, films made of a naturally-occurring substance whichare completely degradable have been attracting considerable attention.

However, currently available biodegradable films made ofnaturally-occurring substances are strictly limited in their use due totheir poor water resistance. Therefore, such materials are virtuallyimpractical for use as containers for foods having a significant watercontent.

ZEIN, a catalogue published by Freeman Industries, Inc. discusses zein,a main protein of corn (Zea mays), as a coating material. Although zeinitself and films of zein prepared in a known manner do not dissolve inwater, zein and films thereof are known to absorb water, and thus becomeweak. Therefore, conventional zein films are not water-resistant andfind little, if any, practical utility.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for preparinga biodegradable film or coating which neither dissolves in water norweakens through water absorption and swelling, and which can be used asa wrapping material for food, or as a coating material for food (e.g.,chocolate, gelatin capsule or enteric agent) or other articles (e.g.,containers for foods).

Another object of the present invention is to provide a method forrendering a biodegradable article water-resistance by coating it withthe biodegradable film described herein.

The above and other objects and advantages in accordance with thepresent invention can be attained by a process for preparing abiodegradable water-resistant film comprising dispersing zein in anaqueous solution comprising acetone in an amount of from 65 to 75% byvolume to form a zein dispersion (which may alternatively becharacterized as a "dispersed system"), forming the zein dispersion intoa film, and drying the film in an atmosphere having a relative humidity(RH) of 85% or less at an initial temperature of 55° C. or less. Thebiodegradable film itself, produced in the above manner, is also a partof the present invention.

The present invention also provides a method for rendering abiodegradable article water-resistant which comprises coating thesurface of a biodegradable article with a zein dispersion prepared bydispersing zein in an aqueous solution comprising 65 to 75% by volumeacetone and drying the coated zein in an atmosphere having a relativehumidity of 85% or less at an initial temperature of 55° C. or less.

DETAILED DESCRIPTION OF THE INVENTION

Zein is a known protein of corn or maize. It can be characterized as aprolamine and contains no tryptophanol, cystine or lysine. According toSwallen, Industrial and Engineering Chemistry., Vol. 33 (3), pp. 394-398(1941), zein is generally considered to be a hydrophobic protein and issoluble in a water-containing organic solvent, e.g., ethyl alcohol oracetone. Further, according to Wilson's cylindrical molecular model(TIBS-July, pp. 306-308 (1984)), zein comprises about 40% of hydrophilicamino acids which are regularly orientated in the molecules to form ahydrophilic region.

The present inventors theorized that both the hydrophobic region andalso the hydrophilic region of zein should be stabilized in order forzein to be dissolved in a water-containing organic solvent, and thatwater provides such a stabilizing effect. As a result of variousinvestigations based on this theory, the inventors have determined thata dispersed system having a specific water to organic solvent ratio cancause an amphipathic protein, such as zein, to be orientated so as tohave a water-resistant structure upon evaporation of the solvent. A filmwith markedly improved water resistance can thus be obtained bymaintaining a dispersed system of an amphipathic substance at such aspecific water/organic solvent ratio for a time sufficient for themolecules to become orientated into a water-resistant structure in whichthe hydrophobic region thereof faces outside with little hydrophilicregion thereof being in contact with water. In other words, thehydrophilic property and hydrophobic property of an amphipathicsubstance can be controlled by purely physical means, such as properchoice of the water to organic solvent ratio of drying conditions, etc.This theory is believed to be responsible for the remarkable effects ofthe invention.

In the present invention, zein is dispersed in a 65 to 75 vol % acetoneaqueous solution to prepare a zein dispersion. A preferred zeinconcentration in the dispersion is from 5 to 20 w/v %. The zein shouldremain in the dispersed system for a time sufficient for the moleculesto become orientated into a water-resistant steric structure.

In order that the zein molecules may remain in the desired stericorientation during drying, drying conditions should be so selected as toachieve well-balanced evaporation of water and acetone. In order toaccelerate evaporation chiefly of water, it is necessary to conductdrying in an atmosphere having a relatively humidity of not more than85%, preferably 20 to 70%. Further, abrupt evaporation of acetone shouldbe avoided by starting the heating from a temperature of not higher than55° C., preferably 20° to 40° C. Under these drying conditions apractical water-resistant zein film can be formed.

The "water-resistant" film as used herein means a film having aconsiderable barrier property against permeation of water as a liquid.In other words, the film does not dissolve in water and hardly suffersfrom disintegration or reduction in physical strength, even when broughtinto contact with water for a relatively long time.

Drying of the coating film may be carried out either under atmosphericpressure or under reduced pressure. Adjustment of these dryingconditions is preferably made by means of a thermo-hygrostat, a pressurereducing apparatus, etc.

The term "biodegradable" film or article as used herein means a film orarticle which is capable of degrading (or being degraded), for example,with a microorganism, and its biodegradability can be confirmed inaccordance with an ordinary digestion test using a digestive enzyme suchas pepsin or α-chymotrypsin.

The present invention will now be illustrated in greater detail withreference to Examples, but it should be understood that the presentinvention is not construed as being limited thereto.

EXAMPLE 1

One gram of zein (ZEIN DP manufactured by Showa Sangyo Co., Ltd.) wasdispersed in 10 ml of a 70 v/v % acetone aqueous solution and heated ina thermostat at 70° C. for 5 min to be completely dissolved. Thedispersion was taken out of the thermostat and cooled to 30° C. Thedispersion was cast on a plastic sheet placed on a hot plate andpreheated to 30° C. over a casting area of 120 cm² in a thermo-hygrostatkept at 25° C. and 70% R H, and the cast film was dried for 6 hours toobtain a transparent film having an average thickness of 70.2 μm.

EXAMPLE 2

A zein film was prepared in the same manner as in Example 1, except thatthe cast film was dried at 28° C. and 20% RH for 3 hours, and then driedat 60° C. and 20% RH for 1 hour to obtain a transparent film having anaverage thickness of 68.8 μm.

COMPARATIVE EXAMPLE 1

A zein film was prepared in the same manner as in Example 1, except thatdrying of the cast film was carried out in a thermo-hygrostat kept at60° C. and 10% RH.

COMPARATIVE EXAMPLE 2

A zein film was prepared in the same manner as in Comparative Example 1,except for replacing the acetone aqueous solution with a 80 v/v %aqueous solution of ethyl alcohol.

COMPARATIVE EXAMPLE 3

The same procedure as in Example 1 was repeated, except for replacingthe acetone aqueous solution with a 80 v/v % aqueous solution of ethylalcohol.

Each of the films prepared was firmly held in place between two siliconO-rings having a diameter of 8 cm, and a water tube having a height of60 cm was placed at the center of the film. Water absorption (a statethat absorbs water but does not become weak) and water swelling (a statethat causes increase in volume, weakening or deformation) of the filmwas observed, and the time required for water to seep through the filmto the reverse side was measured. The results of observation wereevaluated according to the following rating system as shown in Table 1.It should be noted that water resistance of the zein film or the rate ofseeping of water through the zein film varies depending on the filmthickness and is not therefore limited to the results shown in Table 1.

Rating System:

1. . . No change was observed.

2. . . Slight change due to water absorption or swelling was observed.

3. . . Significant change of the film was observed, and water had seepedthrough to the reverse side.

4. . . The film disintegrated.

                  TABLE 1                                                         ______________________________________                                                Exam- Exam-   Compar.  Compar.                                                                              Compar.                                         ple   ple     Example  Example                                                                              Example                                         1     2       1        2      3                                       ______________________________________                                        Film formation                                                                          good    good    excellent                                                                            excellent                                                                            incapable                             Property                                of film                                                                       formation                             Water                                                                         absorption:                                                                   after 5 mins.                                                                           1       1       2      3                                            after 1 hr.                                                                             1       1       3      3                                            after 3 hrs.                                                                            2       2                                                           after 40 hrs.                                                                           3       3                                                           Water swelling:                                                               after 5 mins.                                                                           1       1       1      3                                            after 1 hr.                                                                             1       1       2      3                                            after 3 hrs.                                                                            2       2                                                           after 40 hrs.                                                                           2       2                                                           Seeping of                                                                    water:                                                                        after 5 mins.                                                                           1       1       1      1                                            after 1 hr.                                                                             1       1       3      4                                            after 3 hrs.                                                                            1       1                                                           after 40 hrs.                                                                           3       3                                                           ______________________________________                                    

EXAMPLE 3

One gram of zein was dispersed in 10 ml of a 70 v/v % acetone aqueoussolution and heated in a thermostat at 70° C. for 5 min to be completelydissolved. The dispersion was taken out of the thermostat and cooled to30° C. The dispersion was coated on the entire inner surface of abowl-shaped shaped wafer made from glutinous rice flour (in the form ofa Japanese confectionery called "MONAKA", in which sweetened bean pasteis stuffed) having a thickness of 2.5 mm and an inner surface area of 80cm at a spread thickness of 2.5 ml/80 cm² with a brush. The coated waferwas dried in a thermo-hygrostat kept at 30° C. and 55% RH.

COMPARATIVE EXAMPLE 4

One gram of rein was dispersed in 10 ml of a 80 v/v % ethyl alcoholaqueous solution and heated in a thermostat at 70° C. for 5 min to becompletely dissolved. The dispersion was taken out of the thermostat andcooled to 30° C. The dispersion was coated on the entire inner surfaceof a bowl-shaped wafer of the same type and in the same manner as inExample 3, and the coated wafer was dried in a thermohygrostat kept at60° C. and 10% RH.

35 ml of water was placed into each of the bowl-shaped coated wafers ofExample 3 and Comparative Example 4, and into a non-coated wafer forcomparison, at a temperature of 20° C., and the time for water to seepthrough the wafer was measured. The results obtained are shown in Table2 below.

                  TABLE 2                                                         ______________________________________                                                     Compar.   Untreated                                              Example 3    Example 4 Wafer                                                  ______________________________________                                        15 hrs.      8 mins.   instantaneously                                                               disintegrated                                          ______________________________________                                    

As described above, the film or coating film according to the presentinvention neither dissolves in upon contact with water, nor becomes weakdue to water absorption or swelling on contact with water for anextended period of time. Being made from zein, it is biodegradable.Accordingly, the present invention makes it possible to produce abiodegradable film with which a food product can be directly wrapped, ora food product or other articles can be coated, thus furnishing abiodegradable material meeting the current demand for environmentalconservation and purification.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method for rendering a biodegradable articlewater-resistant consisting essentially of coating the surface of abiodegradable article with a zein dispersion prepared by dispersing zeinin an amount of from 5 to 20% by weight per volume in an aqueoussolution comprising acetone in an amount of from 65 to 75% by volume toform said zein dispersion, and drying the coated zein dispersion in anatmosphere having a relative humidity of 85% or less at an initialtemperature of 55° C. or less.